Nuclear Millimeter Continuum Emission in Radio-Quiet Active Galactic Nuclei: an ALMA Spectral and Variability Study
Abstract
Unresolved millimeter (mm) emission is observed in radio-quiet (RQ) Active Galactic Nuclei (AGN), but its physical origin remains debated. Self-absorbed synchrotron emission from the compact X-ray corona is one of the leading candidates. Previous support for this coronal synchrotron origin has mainly been based on variability and spectral indices in a few RQ AGN, with a lack of (quasi-)simultaneous broadband data for larger samples. We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of nine nearby (z ∼ 0.008−0.064) RQ AGN, obtained quasi-simultaneously at 100, 145, and 233 GHz. Based on power-law fits to the spectral energy distributions, we measure millimeter spectral indices ranging from −0.05+0.09−0.09 to 0.88+0.12−0.11. Furthermore, across two 100 GHz observing epochs per source, separated by 4−11 days, we detect flux density variability of 30−56% (>3σ significance level) in three out of nine sources. Spectral slopes alone are insufficient to constrain the origin of the millimeter emission; however, the observed short-timescale variability and inferred brightness temperature values are consistent with a non-thermal emission mechanism arising from a compact emitting region with a size of Rmm< 103 − 104 Rg, such as the corona or a compact jet. Furthermore, we find a clear correlation between the 100/233 GHz and X-ray flux in our sample, and establish a new linear correlation between 145 GHz and X-ray emission.
Related
- Anisotropic Radiation in Super-Eddington Active Galactic Nuclei: The Responsivity of Broad-Line Regions to Continuum Variability
- Probing the Optical/UV Continuum Variability of Active Galactic Nuclei with the Damped Random Walk Model
- Probing the Optical/UV Continuum Variability of Active Galactic Nuclei with the Damped Random Walk Model
- Origins of the UV Continuum and Balmer Emission Lines in Little Red Dots: Observational Validation of Dense Gas Envelope Models Enshrouding the Active Galactic Nucleus
- Evidence for a spectral steepening of the gamma-ray emission from the Galactic Center ridge
BibTeX
@misc{venselaar2026nuclear,
title = {{Nuclear Millimeter Continuum Emission in Radio-Quiet Active Galactic Nuclei: an ALMA Spectral and Variability Study}},
author = {Sophie M. Venselaar and Claudio Ricci and Yoshiyuki Inoue and Chin-Shin Chang and Akihiro Doi and Noemi Barnier and Taiki Kawamuro and Susanne Aalto and Momoka Hamamoto and Elena Shablovinskaya and Ezequiel Treister},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2610.30286},
eprint = {2610.30286},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2610.30286},
}
RIS
TY - UNPB
TI - Nuclear Millimeter Continuum Emission in Radio-Quiet Active Galactic Nuclei: an ALMA Spectral and Variability Study
AU - Sophie M. Venselaar
AU - Claudio Ricci
AU - Yoshiyuki Inoue
AU - Chin-Shin Chang
AU - Akihiro Doi
AU - Noemi Barnier
AU - Taiki Kawamuro
AU - Susanne Aalto
AU - Momoka Hamamoto
AU - Elena Shablovinskaya
AU - Ezequiel Treister
PY - 2026
DA - 2026/10/21
PB - arXiv
DO - 10.48550/arxiv.2610.30286
UR - https://arxiv.org/abs/2610.30286
AB - Unresolved millimeter (mm) emission is observed in radio-quiet (RQ) Active Galactic Nuclei (AGN), but its physical origin remains debated. Self-absorbed synchrotron emission from the compact X-ray corona is one of the leading candidates. Previous support for this coronal synchrotron origin has mainly been based on variability and spectral indices in a few RQ AGN, with a lack of (quasi-)simultaneous broadband data for larger samples. We present Atacama Large Millimeter/sub-millimeter Array (ALMA) observations of nine nearby (z ∼ 0.008−0.064) RQ AGN, obtained quasi-simultaneously at 100, 145, and 233 GHz. Based on power-law fits to the spectral energy distributions, we measure millimeter spectral indices ranging from −0.05^+0.09_−0.09 to 0.88^+0.12_−0.11. Furthermore, across two 100 GHz observing epochs per source, separated by 4−11 days, we detect flux density variability of 30−56% (>3σ significance level) in three out of nine sources. Spectral slopes alone are insufficient to constrain the origin of the millimeter emission; however, the observed short-timescale variability and inferred brightness temperature values are consistent with a non-thermal emission mechanism arising from a compact emitting region with a size of R_mm< 10^3 − 10^4 R_g, such as the corona or a compact jet. Furthermore, we find a clear correlation between the 100/233 GHz and X-ray flux in our sample, and establish a new linear correlation between 145 GHz and X-ray emission.
ER -